1. Field of the Invention
This invention relates to a map acquisition system including a center unit and a map acquisition unit in a vehicle which are connected via a communication medium. In this system, the map acquisition unit obtains from the center unit map data suitable for navigation. This invention also relates to a navigation apparatus having a medium for obtaining map data from the center unit.
2. Description of the Related Art
A vehicle-mounted navigation apparatus is one known electronic device using map data. Such an apparatus displays on its display a map covering places around the present position and lets the driver know, with map data, which route to the destination to take. Commonly, a storage medium, such as a CD-ROM with recorded map data, is mounted on a navigation apparatus. In such a case, map data read out from a storage medium is used to provide, for example, route guidance.
On the other hand, it has been suggested that a map acquisition unit be installed in a navigation apparatus and that the map acquisition unit obtain map data from the outside. Such a map acquisition unit and a center unit are connected via a communication medium and comprise a map acquisition system. The center unit holds map data; the map acquisition unit obtains from the center unit maps necessary for navigation to the destination. This system has the advantage of being able to use detailed maps which cannot completely be stored on a storage medium such as a CD-ROM. It also has the advantage of being able to quickly respond to the update of maps because all that is necessary to change data for the new data to be sent. Furthermore, the cost of a map storage medium and related configurations can be reduced.
According to Japanese Patent Laid-Open Publication No. Hei8-285613, a portable terminal obtains map data from a center unit via a communication line. This portable terminal is carried by the user and sends the center unit information about the position of a starting point and a destination. The center unit sets a route to the destination and sends the portable terminal a map covering a calculated route.
Because a map acquisition system has limited communication capability, the number of maps obtained at one time should be minimized. In terms of transfer rate and transmission quality, it is difficult to send a large number of maps at one time and memory for holding map data requires a greater capacity for large amounts of data. However, no known art can effectively deal with the above needs.
In a system according to the above publication, for example, a number of maps adjacent to one another along a calculated route are sent. These maps are drawn to the same scale and are similar size. The amount of map data, therefore, increases in proportion to the distance to a destination. A distant destination will lead to a vast amount of data, which increases communication time. Also, processes using map data cannot be started before the communication has been completed. A larger amount of map data also requires a larger amount of memory in a vehicle. To avoid these problems, the above group of maps with the same scale and size can be obtained in parts. It is difficult, however, to perform user-friendly navigation with maps showing only part of a calculated route.
As stated above, furthermore, the cost of storage medium, such as a CD-ROM, and related configurations can be reduced by installing a map acquisition unit, which may lead to a low-cost navigation apparatus. In systems which have been suggested, including that according to the above publication, it is, however, assumed that a center unit sets a route to a destination. Because map data for route guidance is not provided in a vehicle. The center unit must perform the calculation of a route for each of many vehicles, which is a heavy burden on the center unit. To reduce this burden, a vehicle can obtain map data used by the center unit for route calculation. This will, however, increase the amount of data that must be sent, causing communication capability problems. Therefore, a vehicle should be able to calculate a route by receiving a proper amount of data.